1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a laser light source.
2. Description of the Related Art
In an image forming apparatus that deflects a laser beam modulated based on an image signal using a rotary polygon mirror to scan on a photosensitive drum, there have been proposed various techniques for improving accuracy of the scanning position. For example, as a technique of measuring the path of a laser beam in advance for improving accuracy of the scanning position, there has been proposed a technique that corrects the scanning position on a real-time basis using sensors disposed at respective locations outside opposite ends of a photosensitive drum, for detecting the laser beam (see e.g. Japanese Patent Laid-Open Publication No. 2003-21799).
More specifically, in the technique described in Japanese Patent Laid-Open Publication No. 2003-21799, one of the sensors disposed at a location outside one end of the photosensitive drum close to an image writing start position, for detecting the laser beam, is used to adjust the image writing start timing in a main scanning direction of the laser beam to thereby adjust the image writing start position in the main scanning direction. Further, a time period over which the laser beam passes between the sensors disposed at respective locations outside the opposite ends of the photosensitive drum is measured to thereby correct scanning density in the main scanning direction.
In the technique described in Japanese Patent Laid-Open Publication No. 2003-21799, it is possible to calculate the scanning speed of a laser beam passing between the sensors from the difference in times at which the laser beam passes the sensors disposed at respective locations outside the opposite ends of the photosensitive drum. This makes it possible to correct non-uniformity in scanning density due to varying factors, such as rotational fluctuation of a polygon mirror motor, or variation in the processing accuracy of a polygon mirror surface.
However, the varying factors of scanning density are not limited to these, and further includes e.g. wavelength variation of the laser beam. A semiconductor laser chip that oscillates only a single-wavelength laser beam, a so-called single-mode laser beam, is generally used for image forming apparatuses. Therefore, the following description will be given of wavelength variation of a laser beam emitted from a semiconductor laser chip.
In the semiconductor laser chip, the wavelength varies due to a temperature change in the usage environment and a temperature change caused by self-heating. This phenomenon is called “mode hop or mode hopping”. FIG. 13 is a diagram showing an example of changes in wavelength of a laser beam, due to the mode hopping phenomenon. In FIG. 13, reference numeral 101 denotes change characteristics exhibited when the temperature rises, reference numeral 102 denotes change characteristics exhibited when the temperature falls, reference numeral 103 denotes an amount of change in temperature, and reference numeral 104 denotes an amount of change in wavelength.
FIG. 13 shows an example of characteristics of a long resonator-type semiconductor laser chip, and the wavelength changes in a stepped manner at approximately fixed intervals as the temperature becomes higher. Further, it is also known that a change caused by a temperature rise (change characteristics 101) and a change caused by a temperature fall (change characteristics 102) are different from each other.
In an image forming apparatus, a laser beam passes through a plurality of optical lenses before reaching a surface of the photosensitive drum, and hence if the wavelength varies in the laser beam due to a mode hop, an irradiation angle deflects by an amount corresponding to chromatic aberration of each optical lens. Therefore, on the photosensitive drum surface, the irradiation position undergoes a sudden change at the moment of occurrence of a mode hop, and the scanning density changes before and after the change in the irradiation position, which degrades the accuracy of the scanning position. Further, if a mode hop occurs when the sensors for detecting laser beams disposed at respective locations outside opposite ends of the photosensitive drum are receiving the laser beam, to cause a sudden change in the wavelength, it is impossible to accurately measure the scanning speed. Note that it is impractical from the viewpoints of control accuracy and costs to control the temperature of a chip so as to prevent occurrence of a mode hop.
FIG. 14 is a diagram showing another example of changes in the wavelength of a laser beam due to the mode hopping phenomenon. In FIG. 14, reference numeral 105 denotes change characteristics, reference numeral 106 denotes an amount of change in temperature, and reference numeral 107 denotes an amount of change in wavelength. Note that the change characteristics 101 etc. appearing in FIG. 13 are also shown in FIG. 14, for comparison. FIG. 14 shows the characteristics of a short resonator-type semiconductor laser chip, such as a VCSEL.
When the short resonator-type semiconductor laser chip is used, compared with a case of using the long resonator type semiconductor laser chip, although an interval of occurrence of mod hopping with respect to a change in temperature becomes long, an amount of variation in wavelength when a mode hop occurs increases. Therefore, also when the short resonator type semiconductor laser chip is used, there is caused the same problem as caused when the long resonator type semiconductor laser chip is used.